In order to develop secure and reliable digital communication systems, frequency hop nets have been developed. These systems pseudo randomly change their operating band or channel, making unauthorized reception very difficult. Frequency hop nets are able to maintain intelligible communications with as much as 20% of their channels jammed, and for this reason, covert frequency hop (FH) nets usually operate in cluttered spectral regions. In addition, the hop rates are generally fairly low (about 50-500 Hz). Thus, radio stations and other potential interference sources with bandwidths greater than 500 Hz will considerably reduce the sensitivity of fourth law type detectors. Fourth law detectors are detectors whose output signal/noise ratio (SNR.sub.o) is proportional to the fourth power of the input signal/noise ratio (SNR.sub.i); thus SNR.sub.o =K(SNR.sub.i).sup.4, with K being a constant.
Because of this limitation, practical frequency hopping (FH) detection schemes have often utilized channelizers which allow the operator to keep track of narrowband interference sources. These methods rely on the high instantaneous signal-to-noise ratio in the occupied channel for detection. However, channelizers suffer from a lack of efficient automatic detection algorithms. Thus, it would be useful to develop a hybrid detector which would include both channelizing and automatic feature detection circuitry and yet would still retain the advantages of both of these systems.
Frequency hop radios create processing gain by utilizing a large number of independent hop locations. For example, the Jaguar (manufactured by Racal-Tacticom, Ltd.) radio makes use of approximately 256 (minimum, - may use up to 2000) different hop locations. It may be seen that the input bandwidth, W, of a frequency hop detector is much larger than the width of the binary phase shift keyed enVelope (BPSK), B. "B" is the bandwidth of the BPSK envelope and B is assumed to be less than W.
Therefore, the BPSK modulation can be collapsed and the noise decorrelated by a "delay-and-complex conjugate multiply stage" in which the delay is set to approximately 1/W. This method is utilized by the type of hop rate detector known as the MODAC hop rate detector, FIG. 2B. The MODAC detector is manufactured by Pacific Sierra Research, Los Angeles, CA.
The output signal of the MODAC hop rate detector is seen to be a random complex phase shift keying signal (PSK) with transitions occurring at the hop rate, 1/T.sub.h. In this situation, the PSK signal-to-noise ratio is significantly improved by low pass filtering near the hop rate, and a spectral line (at the hop rate) is generated by another delay-and-complex conjugate multiply stage in which the delay is set to approximately T.sub.h /2.
In the AC hop rate detector, shown in FIG. 2A, and in the AC radiometer shown in FIG. 3A, the input band is divided into two "half bands", and the BPSK modulation is collapsed by magnitude squaring. The outputs of the squaring devices are then subtracted to form a bipolar signal. The difference amplifier is AC coupled (eliminating the DC) to the second stage of the detector because of the direct current (DC) component which is generated by the magnitude squaring of the noise.
The input signal hops randomly between the two half bands, and thus the first stage output signal is a random direct sequence (DS) waveform with transitions occurring at the hop rate. As with the MODAC detector, the DS signal-to-noise ratio is significantly improved by lowpass filtering near the hop rate.
The AC radiometer (FIG. 3A) collapses the direct sequence (DS) signal by squaring, and then utilizes an integrator or a lowpass filter for detection. The AC hop rate detector (of FIG. 2A) generates a spectral line at the hop rate with a delay-and-mix circuit, with the delay set to approximately T.sub.h /2, as with the MODAC detector.
Up until the first lowpass filter, the AC hop rate detector and the AC radiometer are identical. However, the AC hop rate detector delay-and-mix circuit (FIG. 2A) generates a square wave with one-half the input signal amplitude, and thus one-fourth the signal power. The power in the fundamental of the square wave is further reduced by a factor of 4/.pi..sup.2.
It follows that the AC radiometer (FIG. 3) output signal-to-noise ratio is approximately 9 dB greater than that of the AC hop rate detector (FIG. 2A).
Additionally, analysis has been made to indicate that, for low input signal-to-noise ratios, the AC hop rate detector (FIG. 2A) outperforms the MODAC hop rate detector (FIG. 2B) by 3 dB.
Spectral analysis techniques, it may be understood, will not always reveal the presence of hybrid FH/DS (frequency hopping/direct sequence) signals because of the inherent covert nature of these signals. However, the class of fourth law detectors described heretofore has been shown to be useful against all types of frequency hopped signals.
For example, the AC radiometer (FIG. 3) generates a DC level when FH signals are "present", thus reducing the signal present/signal absent decision to a comparison with a set threshold. In addition to "signal presence", the "hop rate" can be determined with both the AC hop rate detector (FIG. 2A), and the MODAC hop rate detector (FIG. 2B). Each of these detectors generates a spectral line at the hop rate, which can be detected and characterized by ordinary spectral analysis techniques.
Another class of detectors which has been shown to be useful against frequency hopped signals utilizes channelizing techniques. At any given point in time, the hybrid FH/DS signal is present in one channel only, thus providing a much higher instantaneous signal-to-noise ratio, which can be exploited by various methods.
Thus, it is an object of the present invention to provide for a hybrid channelizing/fourth law frequency hopped signal detector which has near optimal performance in most practical situations.
From FIG. 2A, it is clear that the AC hop rate detector lends itself to channelization, since the input signal is first divided into two separate frequency bands. In particular, the input signal of bandwidth W may be divided into L contiguous bands by a bandpass filter bank (FIG. 3B). Each bandpass filter is followed by a "magnitude squaring" device, which produces a voltage level which is, on the average, the power of the signal and noise present in that frequency band. It is assumed that the filter passbands are large enough that the input signal will usually be contained in a single band, or channel, in which case the output signal will be the same as for the AC hop rate detector. In addition, analysis has shown that the spectral density of the output noise near DC is the same as for the AC hop rate detector, and hence the output signal-to-noise ratio will also be the same. Thus, if L is a large number, then the channels containing narrowband interference can be switched off without significantly affecting the output signal-to-noise ratio in the absence of that interference. Hence it can be seen that the hybrid detector shown in FIG. 11 combines many of the advantages of channelization with the advantages inherent in the AC hop rate detector. However, this hybrid detector design suffers from the drawback that it requires the constant attention of the operator.